Sample preparation and analysis presents many logistical problems. Conventionally, many medical samples (such as blood, saliva, urine and swab eluate) are provided to a doctor, for example a general practitioner doctor (GP) or a principle care physician (PCP), in a local surgery without the equipment necessary to analyse the sample. Hence, the sample must be sent to a laboratory where the sample is analysed. The test results must then be collated and returned to the GP to analyse the results and make a diagnosis. This approach is inadequate. Firstly, there is a significant risk that a sample is lost in transit or mismatched with the wrong patient. Moreover, whilst recent developments in technology have reduced the overall time taken to conduct the test, the delay involved in sending the sample to a laboratory is unsatisfactory.
Nevertheless, analytical systems of the kind found in laboratories are complex and it is often difficult to provide sufficient amounts of pure targets from source samples to reliably perform downstream analytical assays. This typically prohibits local GP surgeries from being able to carry out such tests on site.
However, in recent years efforts have been made to reduce the scale of the analytical systems to make tests faster and simpler to run, and require smaller quantities of sample. For instance, “laboratory on a chip” (LOC) devices (a subset of microfluidic devices) integrate almost all medical tests or diagnostic operations performed in a hospital on a single microfluidic chip. The channels forming such microfluidics devices handle small fluid volumes and are connected together so as to achieve a desired function such as mixing of a sample, moving the sample through the device, reacting the sample with different reagents, and so on. These chips may be inserted into machines to control the performance of a test and measure the results.
However, it has been found that handling a sample in a microfluidics device can be very difficult. In such small channels as are found on a conventional LOC, it is difficult to apply external forces to move the sample from one site to another to perform different actions on the sample. There is also a limit to the complexity of a LOC device which operates purely using capillary action. Furthermore, owing to the small sample sizes of LOC's, the devices have reduced sensitivity and the probability of a target being present in the sample is thus reduced.
An alternative approach is to use a fluidic cartridge. The scale of the components of a fluidic cartridge is larger than for a microfluidic device, and so it becomes possible to move a sample through various different sites to perform different actions on it. This makes it possible to perform more complex tests than may be conducted using typical LOC devices, whilst still providing an analytical system of potential use in a local GP surgery.
Scientific assays useful in medical diagnostics have increasingly involved biochemical procedures, such as the polymerase chain reaction (“PCR”). The PCR assay has provided a powerful method of assaying for the presence of defined segments of nucleic acids. It is therefore desirable to perform a PCR assay on a fluidic cartridge.
Reducing PCR to the microchip level is important for portable detection technologies and high-throughput analytical systems. The method can be used to assay body fluids for the presence of nucleic acid specific for particular pathogens, such as the Chlamydia trachomatis bacterium, HIV or any other pathogenic microbe.
The introduction of commercially available automated DNA amplification assays has allowed more laboratories to introduce these technologies for routine testing of specimens. However, there is a need to improve the fluidic devices used for this purpose.
It is a requirement of any microfluidics device to minimise leakage from valves. Minimising leakage from valves is particularly important in devices which are designed to handle biological samples. This is because any leakage of sample could not only lead to contamination, but may lead to false positives in future test runs. The need to minimise leakage from a microfluidics system is particularly acute in devices which employ PCR technology since the target DNA is amplified and increases the risk of causing false positive results.
Some cartridges may be adapted to perform several steps of sample analysis from introduction of the sample, through mixing and sample preparation, pumping the sample through the device, reacting the sample with different reagents, and processing and detection. In these devices there may be a front end in which sample preparation takes place and a back end in which processing and detection takes place. The front end of the cartridge is typically an open system, i.e. vented to atmosphere, for instance where the sample is introduced. Therefore the front end of the system is the most prone to leakage, and it is important that processed fluid cannot move from the back end of the cartridge, upstream to the front end of the cartridge where leakage may occur. In LOC devices the movement of sample around the cartridge is controlled by mechanically or pneumatically actuated valves.
US20090162864 discloses a biological substance detection cartridge comprising a reaction vessel for reacting a probe with a specific biological substance in a sample solution. The cartridge further comprises a porous membrane facing the inside of the reaction vessel, a gas-liquid separation membrane superposed with the porous membrane and is equipped with an air pump which is provided on the opposite side of the gas-liquid separation membrane from the side contacting the porous membrane, and with which the interior can be kept at negative pressure during the reaction between the biological substance and the probe. This allows any bubbles generated in the reaction vessel during the reaction to be discharged by a simple method through the gas-liquid separation membrane
In addition to minimising leakage of the valve during and after use, it is important in devices used for volume sensitive analysis (such as cartridges using PCR technology) that when valves are moved from their open position to their closed position, they do not force large quantities of surplus liquid resting in the valve chamber back into the system.